(1) Filed of the Invention
The present invention relates to a suspension system for carrying an external load with an aircraft, and it also relates to an aircraft equipped with such a device and to an associated method.
The technical field of the invention is thus the field of systems making it possible to fasten an external load to an aircraft, and in particular to a rotary-wing aircraft.
(2) Description of Related Art
Conventionally, an aircraft has a structure for carrying an external load that is independent of that aircraft, referred to for convenience as a “load-carrier” structure. The load-carrier structure may include fuselage frames of the aircraft, or also a sponson, for example.
The aircraft may then be provided with a suspension system connected to the load-carrier structure in order to carry an external load. The suspension system thus includes a coupling device that is attached under the load-carrier structure.
By way of example, the coupling device may be provided with coupling means of the hook type. A sling is then attached to the hook in order to enable it to carry external loads.
The coupling device may also comprise a fastener device in order to suspend the coupling means from the load-carrier structure. That fastener device may be:
a first device of the universal joint type having two mutually orthogonal pivot axes;
a second device known as a “swing” and comprising a pyramid-shaped framework suspended from the load-carrier structure, e.g. by suspension cables called “pendants”; and
a third device implementing a beam to which the hook is coupled, the beam generally being fastened at at least two points to the load-carrier structure.
On an aircraft having a lift rotor, the suspension system is advantageously arranged near the rotor axis of that lift rotor, in order to limit the disturbances generated by that suspension system on the attitude of the aircraft. In the same manner, the fuel tanks of an aircraft may also be arranged near said rotor axis. Consequently, a suspension system may be located under an aircraft tank during a flight.
Consequently, the suspension system may comprise a return device tending to move the coupling device away from the fuel tanks in the absence of an external load, or from any other member to be preserved. Thus, the return system tends to avoid the risk of the coupling device perforating a sensitive member of the aircraft during a crash.
Such a return device may include at least one resilient member exerting a return force in order to move the coupling device away from a sensitive zone in the absence of an external load. The resilient member may be of the elastic cord type and may for example be connected to the fastener device carrying the coupling means.
When an operator suspends an external load on the suspension system, the resilient member then tends to stretch. The external load thus tends to be aligned vertically under the effect of gravity together with the coupling device, while being positioned in a position called, for convenience, a “working position”.
In contrast, when the external load is removed, the resilient member retracts in order to move the coupling device away from the working position by positioning the suspension system in a position called, for convenience, a “storage position”. By way of example, the storage position is selected so that the coupling device does not pierce a fuel tank in the event of a crash.
The resilient member exerts a considerable return force on the fastener device, in order to hold it in the storage position in spite of the accelerations to which it is subjected when the aircraft is advancing. That return force further increases as a function of the stretching of the resilient member, and therefore as a function of the external load being carried and of the forward speed of the aircraft.
Consequently, that return force is important for holding the coupling device in the storage position. That return force is even greater in the presence of a load, the resilient member then being stretched. Consequently, that resilient member may disturb the equilibrium of the external load carried in the working position by generating a non-negligible return force.
In addition, when the external load is released, that return force leads to the coupling device returning to the storage position suddenly and quickly. Abutments may be installed in order to avoid an impact with the load-carrier structure. However, the returning fastener device may potentially weaken such abutments. In addition, since the return force is considerable, the fastener device is subjected to acceleration that is also considerable and it is returned in such a manner that it reaches its storage position at high speed. That method requires the system of abutments to be able to absorb the energy corresponding to that high impact speed.
In addition, an operator must exert a considerable tractive force on the suspension system in order to fasten an external load suitably so as to overcome the return force exerted by the resilient member. The further the operator moves the coupling device away from its storage position, the greater the force required by the operator.
In order to overcome those drawbacks, resilient members are known that extend over a great length so as to minimize their stiffness.
Suspension systems may include shock absorbers, blocking means, or also devices that activate depending on the position of the fastener device.
Document U.S. Pat. No. 3,044,818 presents a suspension system provided with a fastener device having a pyramid-shaped framework. That pyramid-shaped framework may include a base having three or four sides and bars rising from the base towards an apex of the framework. The coupling means are located under an opening made in a fuselage.
Documents US 2007/256650, U.S. Pat. No. 6,896,230, U.S. Pat. No. 4,736,641, and FR 2 271 924 do not present teaching relating to the drawbacks of suspension systems provided with a resilient return member.
Document US 2007/256650 describes a mechanical tie for a piston engine or a pump.
Document U.S. Pat. No. 6,896,230 describes a hinged arm supporting an article on a desk.
Document U.S. Pat. No. 4,736,641 presents a device for motorizing a gate.
Document FR 2 271 924 describes a parallelogram transmission mechanism for producing a compressive force, in particular in a compactor of household refuse.
Documents US 2007/256650, U.S. Pat. No. 6,896,230, U.S. Pat. No. 4,736,641, and FR 2 271 924 thus do not come within the technical field of suspension systems for aircraft.
Documents JP 2010/280382, U.S. Pat. No. 3,845,937, U.S. Pat. No. 3,227,401, and U.S. Pat. No. 2,364,042 also lie outside the field of the invention.